Wave movement in the sea and in large inland lakes constitutes a potential source of energy that has scarcely been exploited so far. The available wave energy is dependent on the height of the waves, and naturally differs in different locations. The average wave energy during a year is dependent on the various wind conditions, which are greatly influenced by the distance of the location from the nearest coast. Measurements have been carried out in the North Sea, for instance. At one measuring point approximately 100 km west of the coast of Jylland, Denmark, where the depth was about 50 m, wave heights were measured over a long period of time and the available energy was calculated. The following table was obtained:
Height of waveWave periodOutputmsec.kW/mHours/Year<0.5——9661424103251219823632944476644558115211>5.5>145119
Thus, during slightly less than half the time the height of the waves is about 1 m, producing an output of 2 kW/m. However, the most energy is available from wave heights in the region of 2–5 meter, taking into consideration that the output increases greatly with increased wave height.
Various types of wave-power units have been proposed to utilize the energy available from the wave movement in the ocean for generating electric power. However, these have been unsuccessful in competing with conventional production of electric power. Wave-power plants realized so far have been primarily experimental plants or have been used to supply power locally for navigation buoys. If commercial production of electricity is to be possible, thus providing access to the vast reserve of energy in the movement of ocean waves, is it not only necessary to place the units in suitable spots, t is also necessary for the unit to be reliable in operation, highly efficient and low in manufacturing and running costs.
Amongst feasible principles for converting wave-movement energy into electric power, therefore, a linear generator probably satisfies these requirements to the greatest extent.
The vertical movements of the floating body, caused by the wave movement can thus be converted directly to a to-and-fro movement of the rotor of the generator. A linear generator can be constructed to be extremely robust and simple and since it is anchored at the sea bed it will be stable and unaffected by currents in the water. The only movable part of the generator is the rotor moving to and fro. Storage problems are substantially eliminated. The unit will be extremely reliable in operation thanks to its few movable parts and simple construction.
U.S. Pat. No. 6,020,653, for instance, reveals a wave-power unit based on the linear generator principle to be already known. The publication thus describes a generator anchored to the sea bed, which produces electric energy from the wave movement at the surface of the sea. A generator coil is connected to a floating body so that the coil moves up and down with the movements of the waves. A magnetic field acts on the coil when it moves so that an electromagnetic force is generated therein. The magnetic field is such that it produces a uniform field with simple magnetic orientation along the entire stroke length of the coil. The generator comprises a base plate on the sea bed, which supports the magnetic core in which the coil moves.
A wave-power unit provided with an electronic linear generator is also known through U.S. Pat. No. 4,539,485. Its rotor consists of a number of permanent magnets and the winding of the generator is arranged in the surrounding stator. A great drawback is that the stator winding consists of a single coil. It therefore has no poles. This means that the induced current will have extremely low frequency since the linear movement of the rotor is slow.
The object of the present invention is to produce a wave-power unit of the relevant type which to a greater extent fulfils the demands for reliable operation, simplicity and cost effectiveness.